The present invention relates to a constant current source for use in radio frequency circuits. Specifically, a current source having a controllable temperature coefficient is described.
Radio frequency circuit applications for the cellular telephone field may require circuits which can operate over a wide temperature range. In the case of a transmitter circuit for a radio telephone, it is desirable to maintain a power output characteristic constant so that the compression point is stable with temperature. However, temperature changes typically decrease the gain or transconductance of active devices in the circuit, even when current is maintained constant over temperature. The loss in gain will decrease the compression point for an amplifier biased to operate in a class A mode of operation. As the compression point decreases, increased input signal levels do not increase the output signal level proportionally. It may be desirable in some applications to increase the bias current supplied to the amplifier to offset the loss in transconductance using a current source with a controllable temperature coefficient. A current source having a small positive temperature coefficient makes it possible to maintain the device gain and improve the overall stability of the RF circuit gain, noise figure and power output over an operating temperature range.
In accordance with the invention, a current source is provided which has a temperature coefficient which can be invariant with respect to temperature, or which may provide some small selectable temperature coefficient to offset component degradation with temperature. The invention generates a bandgap voltage which is coupled to a current source. The temperature coefficient of the bandgap voltage is selected by the value of a first resistor and the value of a second resistor of the bandgap generator. The bandgap voltage applied to the current source substantially determines the level of current produced by the current source. By controlling the relative resistance values, the temperature coefficient for the current source is also established.